A known air intake system with light weight is disclosed in Japanese Patent Laid-Open Publication No. H11-182367. The known intake manifold disclosed in Japanese Patent Laid-Open Publication No. H11-182367 includes a plurality of curved cylindrical distribution pipes which are unitary formed by injection molding with resin.
Another known air intake system with light weight is disclosed in Japanese Patent Laid-Open Publication No. H06-142905. With the air intake system disclosed in Japanese Patent Laid-Open Publication No. H06-142905, a main body portion is produced by aluminum casting, pipes are produced as individual members with aluminum stretched material, and the body portion and the pipes are assembled by welding.
Notwithstanding, according to the known intake manifold having the distribution pipes unitary formed by injection molding with resin, it is required to prepare a core consumed for producing every product, the core being made of precious low temperature melting metal in order to form cylinders. This increases the manufacturing cost. In addition, it is difficult to ensure the rigidity when the entire intake manifold is made of resin, and thus the resin-made intake manifold is susceptible to be influenced by the engine vibration, which causes large vibration noise.
On the other hand, with the known intake manifold made of aluminum, welding for assembling each member increases the manufacturing cost.
A need thus exists for an air intake system including an intake manifold with light weight and with high rigidity without being provided with a precious core and with low assembly cost.